Thermal Effects of Phase Transformations

Abstract

A very simple set of arguments will convince you that, for the theory of phase transformations to be realistic, we have to consider the OP evolution together with other processes that take place simultaneously with the phase transformations. For instance, as we noted in Chap. 2, a phase transition of the first kind is accompanied by the release of the latent heat, which amounts to the difference of the internal energies (or enthalpies) of the phases on both sides of the transition. The heat does not remain localized at the sites where it was released, usually positions of the interfaces. Due to the mechanism of heat conduction, it will diffuse to the places with lower temperatures causing the temperature field to vary. The redistribution of heat and equilibration of temperature causes the feedback effect on the phase transition in the form of changing rate and in some cases even the direction of the transformation. Another example comes from transformations in systems of varying density, e.g., mixtures or substances in gravitational fields. Such transformations cause flow of matter, which also has a feedback effect on the transformation. Many of these processes have characteristic length and time scales longer than those of the OP variations; that’s why sometimes they are called “hydrodynamics” modes. The questions that we have to answer are: How do we couple the OP evolution to these processes? What physical principles are important here? How do we maintain the thermodynamic (physical) consistency between the descriptions of all processes in the system? The main question, which is discussed in this chapter, is: How can we incorporate the mechanisms of heat release and redistribution into our method in a physically rigorous and consistent way? Another question that we will look at is: What are the new effects or features that we may expect from the transformations that are accompanied by the latent heat release? In this chapter, we review practically all aspects of the book with the energy conservation constraint included. One interesting and counterintuitive conclusion of our analysis is that the thermal effects appear even in transformations that proceed without any latent heat release.